1. Technical Field
The invention relates to the production of a metal boride precursor mixture comprising components which when combined with a carbonaceous component, produce a carbon composite material suitable for use in a cathode or cell lining of an aluminium electrolysis cell.
2. Description of the Prior Art
Metal borides, such as TiB2 are used in an admixture with carbon components to form ramming pastes, cell linings and cathodes for electrolysis cells. Metal borides are known to improve surface wettability of the electrolytic cell components into which they are added. Although titanium diboride is preferred from the point of view of superior performance, it has the considerable disadvantage that it is very expensive.
Titanium diboride is generally formed by combining equal mole quantities of titanium oxide and boron oxide with a reducing agent. Carbon and aluminum, for example, are both known reducing agents. This reduction of the oxides to titanium diboride is also known to be possible at the start-up of an aluminium producing electrolysis cell, therefore producing TiB2 in situ. However, it is known that in situ formation of the metal boride from the precursors proceeds suitably only if the reactants are thoroughly mixed one with the other, and in particular, contact between the metal oxide and the boron compound is required for the production of the metal boride.
Khazai et al. in U.S. Pat. No. 5,160,698 teach a process for producing metal borides using finely comminuted mixtures of reactants. The mixture of particulate reactants comprises: a metal compound, carbon, and a boron source having a particle size of about 200 microns which is heated to convert the mixture to a metal boride having an average particle size of about 0.05 to about 0.5 micron. The metal compound is one which can be converted to a corresponding metal boride by a reaction with carbon and the boron source under controlled conditions and at temperatures from 1200° C. to 2500° C.
Mirtchi et al. in European Patent EP 1 141 446 B1 teach a wettable and erosion/oxidation-resistant carbon composite material for use in the production of aluminium. Mirtchi et al. provide carbon composite materials which may be fashioned into cathode blocks, jointing pastes or the like that are erosion/oxidation resistant and wettable by molten aluminium when used in electrolysis cells while being relatively inexpensive and easy to produce. Mirtchi et al. utilize a precursor mixture that is preferably finely divided having average particle sizes sufficiently small to allow effective reaction of the precursor during the cell start-up to produce corresponding metal borides.
In preparing carbon composite mixtures containing the metal oxide and the boron compound, segregation of the metal oxide and boron compound may occur even when the particles are finely divided. Furthermore, boron oxide seems to have a detrimental effect on the carbon binder used in block production. Thus, mixing and baking is difficult and leads to lower quality products. It is the object of this invention to overcome at least some of the deficiencies in the prior art.